Filtering circuit



Nov. 3 1925.

Filed Sept. 27, 1919 T. c. FRY

FILTERING CIRCUIT 2 Sheets-Sheet 1.

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Thorn/on C. 5

' Nov; 1925- 1,559,864

'r. c. FRY

FILTERING CIRCUIT Filed Sept. 27, 1919 2 Sheets-Sheet 2 Patented Nov. 3, 1925.

UNITED STATES l,1559,864 PATENT OFFIQE.

THORNTON C. FRY, 0F WYOMING, NEW JERSEY, ASSIG-NOR TO WESTERN ELECTRIC COMBANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

l surname omccrr.

Application filed September 27, 1919. Serial No. 326,857.

To all whom it may concern:

Be it known that I, THORNTON C. F RY, a

citizen of the United States, residing at Wyoming, in the county of Essex, State of New Jersey, have invented certain new and useful Improvements in Filtering Circuits, of which the following is a full, clear, concise, and exact description.

The present invention relates to the selective transmission of ranges or hands of frequencies and has for an object the attainment of more sharply defined boundaries for the selected bands or ranges of frequencies than have heretofore been found possible.

The importance of definitely limited bands in all systems in which the bands have an individual significance will be recognized. For example, in multiplex signaling systems in which the individual communication channels are distinguished from one another by the frequencies which the several channels employ, the more definite are the boundaries of the frequency range or hand of each channel, the closer together in terms of frequen cies can the channels he situated Without their interfering with one another, and the greater is the number ofchannels therefore that can be included Within a given available frequency range.

A feature of the invention is the use of low'pass and high-pass filters in combinations to give sharply defined boundaries to the transmitted bands or ranges of frequen-- cies.

By low-pass filter is meant a filter which will transmit with negligibly small attenuation, currents of all frequencies between zero and a predetermined limit, while attenuatingand substantially extinguishing currents of all frequencies above that limit, conversely, a high-pass filter is one that will attenuate and substantially extinguish currents of all frequencies between zero and a predetermined limit, but will transmitwith negligihly small attenuation currents of all fre quencies above said predetermined limit.

The filters that are preferably employed according to the present invention are suppression filters because this oi filter has the property of givinga sharp cutoff hetween the transmitted and the suppressed frequencies. In the patent to 2i. Gampbell, No. 1,227,113 are given examples of a number of hand filters which may he regarded as transmission filters, their object being to transmit with negligible attenuation only a predetermined band or hands of frequencies. Each section of a band filter contains a series impedance element of one character and a shunt impedance element of a difl'erent character. According to the frequency characteristics for which the band filter is designed, the shunt impedance may consist of a single impedance element or a parallel inductance-capacity impedance, while the series impedance I may consist of asingle impedance element or a series inductance-capacity impedance. The function of the suppression filter is the converse of that stated of the hand filter, the object being to transmit freely all frequencies except a predetermined range. In general, and as employed throughout the specification and claims, the term suppression filter is used to designate a filter which consists of a plurality of sections connected in seriatim, each having series impedance and shunt impedance, and in which either or both of the followingconditions are met: (1) The series impedance of a section is equivalent at all frequencies to that, of a capacity element and an inductance element in parallel; (2) the shunt impedance of a. section is equivalent at all frequenciesto that of a capacity element and an inductanceelemerit. in series. A. low-pass or thigh-pass filter, as defined above, may he constructed on the principle of the transmission band filter or the suppression filter. The advantage of the suppression type of lilter for the purposes of; the present invention as ahoye stated, is that the boundary between the ranges of transmitted frequencies and sup pressed frequencies may he made very sharp. The suppression type of filter is more 'iiully described in the patent to (in, i. Qamphell, No. 1,493,600 dated May is,

In the drawings, Figs. 1 and 2 illustrate two Ways of combining suppression filters. Figs. 3 and teach show a manner of com hining succ'essivepairs of suppression filters and Fig. 5 illustrates diagrammatically an application of the principle of the invention to acarrier signaling system.

Figs. 1 and 2 are largely self-explanatory. in Fig. l, the low-pass suppression filter LP has the property of transmitting only applied frequencies which lie between zero and limit N While the high-pass suppression filter Hi? suppresses all freq;uen-- else except those between a predetermined cutoff frequency and infinity.

These properties are indicated on the drawing by a minus sign following N and a plus sign following M. It N M there will he a hand of width hl-M which is transmitted by both filters, and this hand will have a sharply defined upper boundary due to the cutoif of the filter LP and a sharply defined lower boundary due to the cutofi of the filter H The order of the filters in the circuit can he the reverse of that shown just as well, and either arrangement will. give the hand li -dd.

in Fig. 2, the suppression ranges of the filters LP and HP preierahly overlap, that is, the transmission ranges do not meet, leaving a gap of suppressed frequencies in their common output circuit. If, as before, N M the frequencies that are suppressed lie in the sharply defined hand. N-Jil. The same type of filter might he used in both Figs. 1 and 2. As in the case of Fig. 1, it is innneuterial to which terminals or" the circuit the currents are applied. Since the combination in Fig. 2 will. pass all 'l'requencies below M and above N, if M N it passes all frequencies while, as stated, if N lvl it suppresses the band M to N.

In Fig. 3, if a wide range of ifrequencies is introduced at the terminals 1, 2 all will be transmitted to the points 3, 4c except the sharply defined band Q-P and a circuit may be tapped oil at the points 3, l as indicated for conveying the frequencies lower than P and greater than Q for any desired purpose. Of the frequencies present at points 15' 4, all will he transmitted to the points 5, 6- except those lying within the band M to N. If IliQQ, or if N P, there will. be two suppression hands at points 5', with a sharply defined transmitted hand between the supressed ranges. if M and N lie between P and Q. the second pair oi? filters, as indicated in Fig. 3 would of course have no useful function, which meansthat when this condition holds, the pair of n ters which has the narrowest suppression hand should come firstin the circuit.

In Fig. en arrangement of filters is indicated for giving difiierent sharply de f fined hand of transmit-ted frequencies 111 each of the circuits, 8 9, 10 and 11, The high-pass filters HP HP and HP, are placed in series 111 the line and they should he so arranged. that P N lv All" frequencies higher than. M will be transmitted by the filter H? and all. lower frequencies will be suppressed. lit the low pass filter LP, transmits only the frequen cies "from zero to the predetermined limit N which is assumed to he .reatei than 7 4 the hand li -M will be transmitted to the branch circuit 7, 8. In. the case of a: wave signaling system the frequencies transmitted by the branch 7, 8 will preferably he out side the range of those in branch 9, 'lQ-in order to avoid. crosstalk. N is preferably slightly less than N in this case, although the degree to which the value or N can. be made to approach that of N in practice will depend upon the requirements and upon the sharpness of the cutofis o f the filters, for this property can be varied at will, and upon the number of sections in a filter, The range of frequencies transmitted by HP will therefore be from a frequency slightly above the upper cutoil value of LP upwards and it P' N, there will he a sharply de- I fined band of width F-N transmitted through filter to the branch 9, 10. By extension of the principle, any desired. number of hands of frequenciesot the same or of difi'erent widths may be selectively trans mitted to a succession of branch. circuits, such as those indicated. As in the case of the preceding Figures the direction of ap plication of the currents may be the opposite of that described, and branches 7--8, 9-10, and l.1*-12 instead of serving as receiving circuits may he transmitting circuits. It currents of difl'erent frequencies are applied to branches 78, 9-10, etc., discrete bands of frequencies of widths respectively determined. by the combinations of filters used, will appear in the circuit 12 which is now the output circuit. Insteadot' using high-pass filters in series and a low'pass filter in each branch, the reverse the. frequencies from zero to S. will permit the speech waves to pass to the line 16, but will keep all higher frequencies from passing. The high-pass filter HP, will serve to block-the passage of all frequencies in the essential voice range to the circuits 1?, but will freely transmit; all the frequencies lying above a predetermined limit S, which may he made very-close to the limit 5, by virtue of the definite cutoff property of the filters LP, and HR. These frequencies above S may comprise a number oft carrier waves each modulated by speech or some other wave of comparatively low frequency, and as is well known, in case the carrier waves are speech-modulated, there will be present a number oi'hands or ranges oat frequencies, each range comprising frequencies extending from or near the respective carrier frequency in either or both directions toy a limited distance. Allof these bands oi trequencies'are transmittedthrough nuances the coil 18 and they are to'be separately transmitted. to the respective branch circuits 19,20, 21 and 22, For this purpose, a number of low-pass filters LP, to LP are placed in series each branch. 19, 20, etc, contains a high-pass filter MP HP etc, It will be noted that this arrangement is the converse of thatsho'wn in Fig. 4:, but either arrangement is suitable. The upper limit of the frequency range transmitted by each of the filters L1 LP and LP is greater than that of LP so that none of the three filters will obstruct the passage of currents from the coil 18 to the filter LP of any frequency which the filter LP is capable of transmitting. 0f those frequencies fromS upwards which are thus transmitted to the filter LP this filter transmits only the frequencies below the predetermined upper limit S -l-C and only the band of width C, lying between S, and S +C, will therefore be transmitted to the branch circuit 22., venience that all the bands that are to be separated are of equal width, although this is not at all necessary, Of the frequencies from S upwardsthat reach the filter LB only those within the upper predetermined limit S +2+D will be transmitted giving a transmitted range of frequencies between S, and S +2+l1 Of this range only the frequencies above the limit S,+G+D will be transmitted by the filter HP and therefore the range of frequencies transmitted to the branch 21 will consist only of frequencies between S +C+D and S +2+D or a band of width C. D; represents the frequency interval between the successive bands and is assumed to he the same throughout the bandspresent although this is, of course, not a necessary condition. The more sharply defined are the cutofi properties of the filters, the smaller can D be made Without having interference between the-adjacent bands. By extension of the same principle, it is seen that each band C is se arately delivered to its circuit 20, 19, etc, 1nwhich there may be suitable receiving apparatus, not shown.

The circuit 23- may lead to transmitting apparatus employing frequencies higher than S, to be impressed upon the line 15. The usual balancing-net 24 for the line circuit 15 is shown and the coil 18 is indicated as being the usual conjugate ceil for elimimating interference between incoming and outgoing transmissions,

The CllCllliiS; 19, 2G, 21, 22 insteadoi be ing receiving circuits as described might contam sources of currents of different fre quencies higher than S, to be impressed on line 15.- Or the transmitting circuit to which conductors 23 lead may contain filters similarto L39 HR LP Hi1, etc or any other desired selective means.

lit is assumed for con-,

It will be seen that a great many other combinations of high-pass and low-pass filters to secure desired transmitted or suppressed bands are possible in accordance with the principles that have been herein set forth, and it is to be understood that the various circuit arrangements shown are for of said filters.

2. A selective circuit including high-pass.

suppression filters and low-pass suppression filters, the filters of one kind being connect ed in series, each filter of the other kind be ing connected to the output terminals of a different filter of said one kind, the transmission. frequency range of each filter of said other kind overlapping that of the filter to which it is connected,

3. A selective circuit including high-pass suppression filters and low-pass suppression filters, the filters of one kind being connected in series, each filter of the other kind beingz connected to the output terminals of a different filter of said one kind, the transmission rangeof each filter of said other kind overlapping that of the filter to which it is connected, and. the transmission rangeof each of said filters in series being exclusive of a portion of the transmission range of each of the filters which precede it in the series. i v

4-, A selective circuit including; a highpass filter comprising a line includinga plurality of sections, each section comprising an impedance in series with the line and an impedance in shunt of the line, one of said impedances being composed of inductance and capacity so related as to oppose the transmission through the filter of. current of the frequency for which the reactance of said inductance equals the reactance of said capacity, the other impedance opposing the transmission through. saidv filter of all currents of frequencies less than said frequency, and a low-pass filter connected in series with said high-pass filter and having its tran mission range overlapping that of said highpass filter.

5. The circuit arrangement of claim 4:; said low-pass filter comprising a line including a plurality elf-sections, each of which in cludes impedance in series with the line, an impedance in shunt of the line, one of said inipedances comprising inductance and ca pacity so related as to cppose the transmission through said filter of current of ,the frequency for which the reactanee of said inductance equals the reactance of said capacity, the other impedance opposing the transmission through said filter of all currality of high-pass filters and low-pass filters as claimed in claimb each filter of oriekind being in series with a filter of the other kind, the filters of one kind being all connected in series and a branch circuit con-' nected to the output of each filter of the other kind,

7. A selective system comprising a line having a plurality of low-pass supression filters in series in said line, the cutofi frequencies of the respective filters being successively lower, a branch circuit connected to the output terminals of each of said filters, a high-pass filter in said branch circuit having a outed frequency at least as high as that of the low-pass filter next succeeding in the series the low-pass filter to which said branch circuit is connected.

8. A transmission line forconveying currents of different frequencies, a plurality of circuits associated with said line, each circuit conveying currents ot'a diiilerent range of said frequencies, selective means [between said line and said circuits comprising lowpass filters associated with said line in series with one-another, and high-pass filters in said respective circuits each connected to the output of one of 'said low-pass filters and having a transmission frequency range overlapping that of the low-pass filter to which it is connected.

9. A high. frequency composite signaling line for transmitting currents oiispecch frequencies and currents of frequencies outside the range oi speech "frequencies, a low-pass suppression filter in series with; line for passing only currents including the highest essential speech frequency and lower iiresaid pain.

cuits being connected to the output of each of said low-pass filters which are in series and high-pass suppression filters in the respective branch circuits, the cutoff frequency of each of which is at least as high as that of the next succeeding low-pass filter in the ser1es.

10. Means for transmitting currents of a band of frequencies comprising a high pass filter and a low pass filter in series having overlapping frequency transmission ranges, at least one of said filters being a suppres:

sion filter whereby the transmitted hand of frequencies'is sharply defined at least at one boundary.

11. A plurality of pairs of suppression filters one filter of each air transmitting only currents of frequencies higher than a certain definite cut-oil value and the other transmitting currents of frequencies only below a certain definite cut -ofi value, a circuit common to all of said pairs of filters for conveying currents of frequencies transmitted by each of said filters, and a branch circuit individual to each ofsaid pairs of filters for conveying currents of only the frequem ciesthat' are transmitted by the filters of in witness whereof, Fl hereunto subscribe my name this 26th day oil September A 1)., i919 rnonnron c. car. 

